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Caves are naturally occurring hollow spaces found all over the earth. Although no one knows for sure how many caves there are in the world, there is a pretty good chance that there’s one near youAn image of a cave mouth.
Have you ever wanted to be a spelunker? Caving, also known as spelunking, refers to the exploration of caves as a hobby. It turns out, the act of visiting caves for fun is a popular enough activity that it earned its own cool nickname! Before you go spelunking for the first time, here are some things you need to know. 

How Are They Formed?

Like many landforms, caves form over long periods of time– sometimes hundreds of thousands of years! Caves can form almost anywhere deep within the crust. This doesn’t just mean underground, however. Caves can form underwater, too, or under ice when they form within glaciers.

An image showing the view from inside some caves made of ice.

A miniature cave, made out of ice.

By Water Erosion

Limestone, a type of sedimentary rock, is eroded by water over hundreds of thousands of years in order to make a cave. This water erosion can occur with moving water, such as when waves lap up against the side of a cliff over and over again. It can also occur when rainwater or melted snow or ice soak into the earth.

In these cases, the water absorbed into the earth mixes with carbon dioxide in the soil, resulting in carbonic acid. This mixture continues down into the earth, eating away at limestone and forming channels, which become wider and wider as time passes. Caves formed in this way, made of limestone, marble, and gypsum, are karst caves. 

An image showing marble sea caves carved in the side of a cliff.

A set of karst caves, hollowed out into marble.

Water isn’t the only force that forms caves, though. Lava, molten rock often associated with volcanoes, can erode rock as well. As lava flows, part of the lava moves faster than the rest, usually in the middle. When the lava begins to cool, the slower moving outside lava does so first.

This creates a tube, like a garden hose, in which the cooled lava acts as the passage the faster lava travels through. When the faster-moving lava finally runs out, the tube is empty and hollow, like when a garden hose runs out of water.  

An image of volcanic caves, formed by lava.

A pathway carved into hollow igneous rock.

Stalactites and Stalagmites 

One feature that makes caves both captivating and eerie is the presence of speleothem. Speleothem refers to the mineral deposits that look like icicles inside of a cave. There are a couple of different types of speleothem.
Stalactites are speleothem that hang down from the ceiling of a cave. They form when water drips from cracks in the ceiling. This water is mineral rich, and as it drips it leaves behind calcite. Over time, it forms a cone shape. Stalactites can be long and thin, but can also grow to be very wide, too. 

Stalagmites are like mirror images of stalactites. Rather than hanging from the ceiling, they appear to have grown out of the cave floor! Stalagmites form from mineral-rich drips coming from stalactites. Columns can also form when stalactites and stalagmites grow together.  

An image showing stalactites, stalacmites, and columns.

Stalagmites and stalactites. Which is which? Remember, stalactites cling, “TITE”, to the ceiling.

Mammoth Cave National Park

Spanning 400 miles and growing, Mammoth Cave is a national park in Kentucky. It is the longest discovered cave system in the world. Although it was not named a national park until 1941, we’ve been exploring this incredible landform for thousands of years! The remains of Native Americans dating back to pre-Columbian times have been found within the cave system.
The constant conditions within the cave preserved the remains well. The cave was first re-discovered sometime around the year 1800, according to legend, by a settler named John Houchins. In the centuries that followed, the cave system had many different owners.
For a time, people mined and sold gunpowder from the cave. African American slaves explored the caves and gave tours. People suffering from tuberculosis lived inside of the caves in hopes of recovering. Today, Mammoth Cave is a United States National Park, a World Heritage Site, and a great place to go spelunking

Other Great Resources:

Bill Nye on Caves: https://app.schooltube.com/video/7e90d2f985894d8b9070/Caves_-_Bill_Nye

More on Mammoth Cave: https://www.nps.gov/maca/index.htm

PBS Nova on Cave Formation: http://www.pbs.org/wgbh/nova/caves/

US Government on the Geology of Caves: https://www.nature.nps.gov/Geology/usgsnps/cave/cave.html

 Written By: Caroline Birt